1. Technical Field
This invention relates to a system and a method for liquid silicon containment, such as during the casting of high purity silicon used in solar cells or solar modules.
2. Discussion of Related Art
Photovoltaic cells convert light into electric current. One of the most important features of a photovoltaic cell is its efficiency in converting light energy into electrical energy. Although photovoltaic cells can be fabricated from a variety of semiconductor materials, silicon is generally used because it is readily available at reasonable cost, and because it has a suitable balance of electrical, physical, and chemical properties for use in fabricating photovoltaic cells.
In a known procedure for the manufacture of photovoltaic cells, silicon feedstock is doped with a dopant having either a positive or negative conductivity type, melted, and then crystallized by pulling crystallized silicon out of a melt zone into ingots of monocrystalline silicon (via the Czochralski (CZ) or float zone (FZ) methods). For a FZ process, solid material is fed through a melting zone, melted upon entry into one side of the melting zone, and re-solidified on the other side of the melting zone, generally by contacting a seed crystal.
Recently, a new technique for producing monocrystalline or geometric multicrystalline material in a crucible solidification process (i.e. a cast-in-place or casting process) has been invented, as disclosed in U.S. patent application Ser. Nos. 11/624,365 and 11/624,411, and published in U.S. Patent Application Publication Nos.: 20070169684A1 and 20070169685A1, filed Jan. 18, 2007. Casting processes for preparing multicrystalline silicon ingots are known in the art of photovoltaic technology. Briefly, in such processes, molten silicon is contained in a crucible, such as a quartz crucible, and is cooled in a controlled manner to permit the crystallization of the silicon contained therein. The block of cast crystalline silicon that results is generally cut into bricks having a cross-section that is the same as or close to the size of the wafer to be used for manufacturing a photovoltaic cell, and the bricks are sawn or otherwise cut into such wafers. Multicrystalline silicon produced in such manner is composed of crystal grains where, within the wafers made therefrom, the orientation of the grains relative to one another is effectively random. Monocrystalline or geometric multicrystalline silicon has specifically chosen grain orientations and (in the latter case) grain boundaries, and can be formed by the new casting techniques disclosed in the above-mentioned patent applications by melting in a crucible the solid silicon into liquid silicon in contact with a large seed layer that remains partially solid during the process and through which heat is extracted during solidification, all while remaining in the same crucible. As used herein, the term ‘seed layer’ refers to a crystal or group of crystals with desired crystal orientations that form a continuous layer. They can be made to conform to one side of a crucible for casting purposes.
In order to produce high quality cast ingots, several conditions should be met. Firstly, as much of the ingot as possible should have the desired crystallinity. If the ingot is intended to be monocrystalline, then the entire usable portion of the ingot should be monocrystalline, and likewise for geometric multicrystalline material. Secondly, the silicon should contain as few imperfections as possible. Imperfections can include individual impurities, agglomerates of impurities, intrinsic lattice defects and structural defects in the silicon lattice, such as dislocations and stacking faults. Many of these imperfections can cause a fast recombination of electrical charge carriers in a functioning photovoltaic cell made from crystalline silicon. This can cause a decrease in the efficiency of the cell.
Many years of development have resulted in a minimal amount of imperfections in well-grown CZ and FZ silicon. Dislocation free single crystals can be achieved by first growing a thin neck where all dislocations incorporated at the seed are allowed to grow out. The incorporation of inclusions and secondary phases (for example silicon nitride, silicon oxide or silicon carbide particles) is avoided by maintaining a counter-rotation of the seed crystal relative to the melt. Oxygen incorporation can be lessened using magnetic CZ techniques and minimized using FZ techniques as is known in the industry. Metallic impurities are generally minimized by being segregated to the tang end or left in the potscrap after the boule is brought to an end.
Known containment devices use metal or copper shields in attempts to rapidly dissipate the heat of fusion and solidify the molten silicon. These containment systems result in damage to the copper as some metal still melts and contaminates the silicon rendering it unfit for reuse.
However, even with the above improvements in the CZ and FZ processes, there is a need and a desire to produce high purity crystalline silicon that is less expensive on a per volume basis, needs less capital investment in facilities, needs less space, and/or less complexity to operate, than known CZ and FZ processes. There is a need and a desire for a safer process to cast silicon and provide containment of liquid silicon in the event of a breach. There is also a need and a desire for an apparatus that can be reused following a breach. There is also a need and a desire to be able to reuse the contained silicon in future casting cycles.